The present invention relates to organic electroluminescent (EL) devices, also known as organic light-emitting diodes (OLED) that emit color light.
In color or full-color organic electroluminescent (EL) displays (also known as organic light-emitting diode devices, or OLED devices) having an array of colored pixels such as red, green, and blue color pixels (commonly referred to as RGB pixels), precision patterning of the color-producing organic EL media are required to produce the RGB pixels. The basic OLED device has in common an anode, a cathode, and an organic EL medium sandwiched between the anode and the cathode. The organic EL medium can consist of one or more layers of organic thin films, where one of the layers is primarily responsible for light generation or electroluminescence. This particular layer is generally referred to as the emissive layer or light-emitting layer of the organic EL medium. Other organic layers present in the organic EL medium can provide electronic transport functions primarily and are referred to as either the hole-transporting layer (for hole transport) or electron-transporting layer (for electron transport). In forming the RGB pixels in a full-color OLED display panel, it is necessary to devise a method to precisely pattern the emissive layer of the organic EL medium or the~entire organic EL medium.
In commonly assigned U.S. Pat. No. 5,937,272, Tang has taught a method of patterning multicolor pixels (e.g. red, green, blue subpixels) onto a thin-film-transistor (TFT) array substrate by vapor deposition of an EL material. Such EL material is deposited on a substrate in a selected pattern via the use of a donor coating on a support and an aperture mask.
Using an unpatterned donor sheet and a precision light source, such as a laser, is another method of radiation transfer. Such a method is disclosed by Littman in U.S. Pat. No. 5,688,551, and in a series of patents by Wolk et al. (U.S. Pat. Nos. 6,114,088; 6,140,009; 6,214,520; and 6,221,553). While this is a useful technique for manufacturing, EL devices that include emissive layers prepared this way often suffer from decreased stability relative to El devices with emissive layers prepared in other ways, e.g. vapor deposition.
It is an object of the present invention to provide a way to form an emissive layer using a radiation transfer process from a donor element and to improve the stability of the thus formed emissive layer.
This object is achieved by a method of forming an organic light-emitting device with improved stability, comprising:
a) forming an anode over a substrate;
b) providing a cathode spaced from the anode;
c) providing a donor element including light-emitting material and positioning such donor element in a material-transferring relationship with the substrate;
d) illuminating the donor element with radiation to cause the transfer of light-emitting material to deposit the light-emitting material to form an emissive layer over the anode; and
e) forming an organic layer including an organic compound doped with a low work function metal or metal compound capable of acting as a donor dopant between the emissive layer and the cathode to lower the electron-injecting barrier from the organic layer into the emissive layer thereby improving the stability of the organic light-emitting device.
It is an advantage of this invention that an OLED device manufactured by radiation transfer of a light-emitting material has improved operational stability. It is a further advantage of this invention that the operating voltage at a given current density is reduced.